a) Field of the Invention
The invention is directed to an arrangement for generating extreme ultraviolet (EUV) radiation based on a plasma that is generated by electric discharge in which electrodes are coated with a coating material which is at least partially sacrificed in the local evaporation of emitter material induced by an energy beam and subsequent generation of plasma by electric discharge between the electrodes and which can be renewed by a continual movement of the electrodes in that a vessel containing a molten metal for regenerating the coating and for electrically contacting the electrodes with a pulsed high-voltage source is provided for each electrode. The invention is provided particularly for the production of long-lasting EUV radiation sources for semiconductor lithography.
b) Description of the Related Art
A method known from the prior art and described, e.g., in WO 2005/025280 (DE 103 42 239 A1) uses rotating disk electrodes. The electrodes are immersed peripherally in a bath of molten metal emitter material so that they are wetted in continuous rotation and a thin film of the molten metal adheres to them. A portion of the molten metal film is evaporated by a laser beam for subsequent ignition of the electric discharge, wherein additional portions of the molten metal film are evaporated or expelled as melt. For this reason, the molten metal film can be referred to as a sacrificial layer on the electrodes.
This regenerative application of molten metal ensures substantially identical evaporation conditions for each laser pulse, but has the following disadvantages:                (a) Due to the fact that the disk electrodes are arranged close together, a large portion of the energy emitted by the plasma as electromagnetic radiation and particle radiation is absorbed in comparatively small volumes of the electrodes so that these electrodes are heated to a considerable degree.        (b) Owing to the short dwell period of the electrodes in the molten metal, the cooling of the electrodes in the melt is insufficient so that an additional liquid cooling is required in the interior of the electrodes, for which purpose vacuum feedthroughs which have only a limited lifetime must be used for supporting and driving the rotary electrodes.        (c) A combination of rotary electrodes with injection of liquid or solid material into the discharge zone for a mass-limited supply of emitter material (described, e.g., in DE 10 2005 030 304 A1 which was not previously published) is only possible by means of a tangential trajectory because of the narrow gap between the closely adjacent disk electrodes so that the resulting long route is detrimental to a good pulse-to-pulse stability.        (d) Due to the geometric boundary conditions, only multiply nested reflection optics with grazing beam incidence can be used to bundle the radiation emitted from the plasma, which sharply limits the usable solid angle of the emitted radiation.        